Most subscribers are coupled to a telecommunications network with a twisted pair wire loop commonly referred to as the subscriber loop. Where high speed data signals are to be sent to the subscriber, this twisted pair connects to a node associated with an optical distribution network which can be some 1000 m away from the subscriber. Such an arrangement is depicted in FIG. 1. The first 950 m or so of cabling 12 from a junction box is covered underground with trunking containing many twisted pairs of underground feeders 14, and the final 50 m or so of wire from a distribution point 16 to a subscriber's installation is normally above ground arid consists of an individual twisted pair 18 to each house 20.
In twisted pair copper loop networks, signals are transmitted in differential mode; that is to say, the signals are transmitted along both wires and any Radio Frequency Interference RFI will be picked up by both wires approximately equally with the wanted data signal being determined by the signal between wires at the receiver. Since high speed data is transmitted in this fashion, there should be no transmission problems in such an arrangement. However, there will always be present a certain amount of breakthrough between the common mode and the differential mode and there will always be some interference leaking through differentially, even though the interference is predominantly received in common mode. Typically this differential interference signal may be 30 dB less than the common mode interference signal, but this can cause problems if it is strong enough to distort the output of the associated analogue-to-digital converter, ADC, in addition to providing an increase in the background interference resulting in a corresponding decrease in the output carrier-to-noise ratio, CNR.
Adjacent radio amateurs provide one form of RFI, transmitting in any of the three amateur bands under 10 MHz who are known to cause up to 0 dBm signals to appear differentially on nearby overhead telephone lines. The interference will cause compression problems for the subscriber's ADC which will require the AGC to decrease its gain, causing a loss of dynamic range of signal. Twisted pair wires were originally developed to carry 3 KHz bandwidth; With the advent of services such as video on demand and computer networking, there is a requirement for a larger bandwidth, the provision of which is hampered by the effects of interference as described above.
There are many digital modulation schemes that have been developed, e.g. QAM/CAP, and can be used for subscriber loop schemes as described above. However, all these modulation schemes will be severely affected by interference, especially by high level narrow band interference eg radio amateur transmissions. The interference can cause automatic gain control (AGC) and ADC compression problems if high enough in power, as well as also decreasing the available carrier to noise ratio. Traditional techniques have been based on simple digital filtering techniques in the demodulator which cannot address the AGC/ADC problem. In contrast, analogue cancellation methods benefit from the use of a bandpass filter centred on or near the interferer and of approximate bandwidth to the interferer, which can then be used to determine a feedback loop. In most digital modulation schemes (other than digital multitone (DMT)) this is not readily available.
As a result of interference induced on a transmission line, it may be necessary to cease using channels which are corrupted by the interference, or alternatively to provide additional coding on the affected channels to better protect them from corruption. Both of these methods reduce the bandwidth which is available for the transmission of data.